Existing image-forming apparatuses are known to experience a phenomenon called ghosting in which the history of image formation appears as afterimages in subsequent image formation. For example, when a halftone image is formed after formation of a high-density solid image, the trace of the solid image often appears in the halftone image, resulting in ghosting. Also known is a phenomenon in which the density at a back end of an image is low when a high-density solid image is formed.
In order to suppress ghosting and the decrease in density at the back end of a solid image, a supply roller (toner supplying member) that contacts a developing roller (toner bearing member) and supplies and removes the toner is widely employed in typical development devices. That is, ghosting can be suppressed by erasing the history of the image formation remaining on the developing roller by using the removing action of the supply roller. The decrease in density at the back end of a solid image can be suppressed by supplying new toner from the supply roller to the developing roller.
However, a development device not quipped with the supply roller has been proposed to reduce the size and cost of the development device.
PTL 1 and PTL 2 each propose a development device that includes a toner bearing member employing a structure in which dielectric portions are scattered on its surface but that does not include a supply roller. That is, the dielectric portions on the surface of the toner bearing member are rubbed with a toner layer thickness regulating member (regulating member) either directly or with the toner therebetween so as to charge the dielectric portions and form microfields between the dielectric portions and surrounding regions thereof. The toner transported to the surface of the toner bearing member receives gradient force from the microfields and becomes attracted to the surface of the toner bearing member, and thus the toner bearing member bears the toner. In PTL 1 and PTL 2, it is described that in order to stabilize the charge amount of the toner on the toner bearing member, the position of the dielectric portions in a triboelectric series should be on the polarity side, relative to the regulating member, opposite to the normal charge polarity (charge polarity for developing electrostatic latent images) of the toner. For example, when the toner is negatively charged, the triboelectric series should be (−) toner<regulating member<dielectric portions (+).